Up to now, a method of measuring optical characteristics of a substance to thereby estimate an unknown component in the substance has been known. For example, JP-A 2001-141602 discloses a method of estimating an unknown component, including obtaining a birefringent phase difference (retardation) from the transmitted light intensity when light is transmitted through a substance as a measurement target, and calculating an index of birefringence inherent in the substance from the birefringent phase difference. Birefringence refers to a phenomenon in which two refracted lights appear when light enters an anisotropic medium. A birefringent phase difference is expressed by the product of an index of birefringence and a transmitted light path length. Hence, if the transmitted light path length of the light is different even if the birefringent phase difference is the same, the index of birefringence is also different. Accordingly, an accurate index of birefringence can be obtained using a transmitted light path length accurately determined for a measured birefringent phase difference.
It is unfortunately difficult to obtain an accurate index of birefringence, in the case where the transmitted light path length cannot be easily obtained for a measurement target, for example, in the case where the shape of the measurement target is complicated. Further, in the case where the measurement target is a biological membrane such as the retina of an eye, the measurement target cannot be cut out of a human body, and hence its thickness, that is, the transmitted light path length cannot be measured.
Further, a method of estimating an unknown component in a substance from Fourier spectral characteristics, which are optical characteristics other than birefringent properties, has been known. As described above, birefringent properties are optical characteristics observed when a substance is an anisotropic medium, and hence it is effective to measure both birefringent properties and Fourier spectral characteristics in estimating components of an unknown substance. Unfortunately, conventional devices are not capable of measuring both Fourier spectral characteristics and birefringent properties at the same time.